Non-spin Flip Research Articles

Quantum modeling of interfacial scattering effect on tunneling conductance spectra of magnetic tunnel junction with ferromagnet/insulator/insulator/ferromagnet

The tunneling conductance spectroscopy of ferromagnet/insulator/insulator/ferromagnet junctions with parallel (P) and anti-parallel (AP) alignments of the magnetization directions were theoretically studied based on a scattering method in a one-dimensional system. Due to the junctions in a real experiment being composed of, for example, Fe/ZnO/MgO/Fe materials, in this work the two ferromagnetic electrodes were modeled to be the same, but the two insulating sheets were set to be distinguishable. As the main area of interest was to focus on the effects of the interfacial scattering on the transport behavior, the potential strength at the three interfaces was included in both non-spin–flip (Z) and spin–flip (Zf) scattering based on the Dirac-delta potential model, and all of them were set to be different in magnitude. This means that there are Z1(2)(3) and Zf1(2)(3) for the first, second, and third interfaces. It was found that the potential strength at the first interface (Z1) plays a crucial role in determining the direct transmission or oscillation behavior. For the effect of Zf , an increase in Zf1 causes the oscillatory behavior to increase, while the direct transmission is most prominent when Zf2 or Zf3 increases. For the effect of the insulating band gaps (Ui), the condition for direct transmission is U2≥3U1, where U1(2) is the insulating band gap of the first and second insulators, respectively. Finally, if the electron effect masses of all materials in the junction are approximately equal, then the direct transmission process can occur.

Probing Flat Band Physics in Spin Ice Systems via Polarized Neutron Scattering.

In this Letter, we illustrate how polarized neutron scattering can be used to isolate the spin-spin correlations of modes forming flat bands in a frustrated magnetic system hosting a classical spin liquid phase. In particular, we explain why the nearest-neighbor spin ice model, whose interaction matrix has two flat bands, produces a dispersionless (i.e., "flat") response in the non-spin-flip (NSF) polarized neutron scattering channel and demonstrate that NSF scattering is a highly sensitive probe of correlations induced by weak perturbations that lift the flat band degeneracy. We use this to explain the experimentally measured dispersive (i.e., nonflat) NSF channel of the dipolar spin ice compound Ho_{2}Ti_{2}O_{7}.

Theory of single-charge exchange heavy-ion reactions

The theory of heavy ion single charge exchange reactions is reformulated. In momentum space the reaction amplitude factorizes into a product of projectile and target transition form factors, folded with the nucleon-nucleon isovector interaction and a distortion coefficient which accounts for initial and final state ion-ion elastic interactions. The multipole structure of the transition form factors is studied in detail for Fermi-type non-spin flip and Gamow-Teller-type spin flip transitions, also serving to establish the connection to nuclear beta decay. The reaction kernel is evaluated for central and rank-2 tensor interactions. Initial and final state elastic ion-ion interaction are shown to be dominated by the imaginary part of the optical potential allowing to evaluate the reaction coefficients in the strong absorption limit, realized by the black disk approximation. In that limit the distortion coefficient is evaluated in closed form, revealing the relation to the total reaction cross section and the geometry of the transition form factors. It is shown that at small momentum transfer distortion effects reduce to a simple scaling factor, allowing to define reduced forward-angle cross section which is given by nuclear matrix elements of beta decay-type. The response function formalism is used to describe nuclear charge changing transitions. Spectral distributions obtained by a self-consistent HFB and QRPA approach are discussed for $\tau_\pm$ excitations of $^{18}O$ and $^{40}Ca$, respectively, and compared to spectroscopic data. The interplay of nuclear structure and reaction dynamics is illustrated for the single charge exchange reaction $^{18}O+^{40}Ca \to ^{18}F+^{40}K$ at $T_{lab}=270$ MeV.

Explaining the singlet complexes detected for the reaction Zr(3F) + CH3CH3 through a non-spin flip scheme.

Energy profiles for the lowest lying triplet and singlet electronic pathways that link the reactants Zr + CH3CH3 with the products observed under matrix-isolation conditions were obtained from DFT and CASSCF-MRMP2 calculations. The insertion of the metal into the C-H bond of the organic molecule to yield the oxidative addition product is not favorable for any of the investigated channels. However, the inserted structure H-Zr-CH2CH3 can be obtained from two sequential reactions involving the radical species ZrH and CH2CH3. According to this scheme, a first reaction produces the radical fragments from the ground state of the reactants. Then, the radicals can recombine themselves in a second reaction to form the inserted species H-Zr-CH2CH3. As the triplet and singlet radical asymptotes ZrH + CH2CH3 that vary only in spin of the non-metallic fragment are degenerate, the rebounding of the radicals can occur through both multiplicity channels. It is shown that the low spin channel leads to the most stable structures of the dihydride ZrH2-(CH2)2 and the vinyl metal trihydride complexes ZrH3-CH=CH2 experimentally determined for this reaction under matrix-isolation conditions. The description attained for this interaction does not invoke interactions between the triplet and singlet electronic states emerging from the reactants, as proposed by other authors.

Spin-analyzed SANS for soft matter applications

The small angle neutron scattering (SANS) of nearly Q-independent nuclear spin-incoherent scattering from hydrogen present in most soft matter and biology samples may raise an issue in structure determination in certain soft matter applications. This is true at high wave vector transfer Q where coherent scattering is much weaker than the nearly Q-independent spin-incoherent scattering background. Polarization analysis is capable of separating coherent scattering from spin-incoherent scattering, hence potentially removing the nearly Q-independent background. Here we demonstrate SANS polarization analysis in conjunction with the time-of-flight technique for separation of coherent and nuclear spin-incoherent scattering for a sample of silver behenate back-filled with light water. We describe a complete procedure for SANS polarization analysis for separating coherent from incoherent scattering for soft matter samples that show inelastic scattering. Polarization efficiency correction and subsequent separation of the coherent and incoherent scattering have been done with and without a time-of-flight technique for direct comparisons. In addition, we have accounted for the effect of multiple scattering from light water to determine the contribution of nuclear spin-incoherent scattering in both the spin flip channel and non-spin flip channel when performing SANS polarization analysis. We discuss the possible gain in the signal-to-noise ratio for the measured coherent scattering signal using polarization analysis with the time-of-flight technique compared with routine unpolarized SANS measurements.

The First Asymmetry Measurements in High-Energy Polarized Proton-Nucleus Collision at PHENIX-RHIC

The single spin asymmetries in very forward neutron production had been first observed about a decade ago at RHIC in transversely polarized proton + proton collision at √s = 200 GeV. Although neutron production near zero degrees is well described by the one-pion exchange (OPE) framework, the OPE appeared to be not satisfactory to describe the observed analyzing power A N . The absorptive correction to the OPE generates the asymmetry as a consequence of a phase shift between the spin flip and non-spin flip amplitudes. However the amplitude predicted by the OPE is too small to explain the large observed asymmetries. Only the model which introduces interference between major pion and small a 1 -Reggeon exchange amplitudes has been successful in reproducing the experimental data. During RHIC Run-15, RHIC delivered polarized proton collisions with Au and Al for the first time, enabling the exploration of the mechanism of transverse single-spin asymmetries with nuclear collisions. A very striking A-dependence was discovered in very forward neutron production at PHENIX in transversely polarized proton + nucleus collision at √s = 200 GeV. Such a dependence has not been predicted from the existing framework which has been succesful in proton + proton collision. In this report, experimental and theoretical efforts are discussed to disentangle the mysterious A-dependence in the very forward neutron asymmetry.

Using a non-spin flip model to rationalize the irregular patterns observed in the activation of the C-H and Si-H bonds of small molecules by CpMCO (M = Co, Rh) complexes.

The activation of the C-H and Si-H bonds of CH(CH3)3 and SiH(CH3)3 molecules by organometallic compounds CpMCO (M = Co, Rh) has been investigated through DFT and CASSCF-MRMP2 calculations. In particular, we have analyzed the pathways joining the lowest-lying triplet and singlet states of the reactants with the products arising from the insertion of the metal atom into the C-H or Si-H bonds of the organic molecules. Channels connecting the reactants with the inserted structure Cp(CO)H-M-C(CH3)3 through the oxidative addition of the C-H bond of the organic molecule to the metal fragment were found only for the reaction CpRhCO + CH(CH3)3. However, inserted structures could also be obtained for the interactions of SiH(CH3)3 with CpCoCO and CpRhCO by two sequential reactions involving the formation and rebounding of the radical fragments Cp(CO)H-M + Si(CH3)3. According to this two-step reaction scheme, the complex CpCoCO is unable to activate the C-H bond of the CH(CH3)3 molecule due to the high energy at which the radical fragments Cp(CO)H-M + C(CH3)3 are located. The picture attained for these interactions is consistent with the available experimental data for this kind of reaction and allows rationalization of the differences in the reactivity patterns determined for them without using spin-flip models, as has been proposed in previous studies.

The First Transverse Single Spin Measurement in High Energy Polarized Proton-Nucleus Collision at the PHENIX experiment at RHIC

Large single spin asymmetries in very forward neutron production seen using the PHENIX zero-degree calorimeters are a long established feature of transversely polarized proton-proton collisions at RHIC. Neutron production near zero degrees is well described by the one-pion exchange framework. The absorptive correction to the OPE generates the asymmetry as a consequence of a phase shift between the spin flip and non-spin flip amplitudes. However, the amplitude predicted by the OPE is too small to explain the large observed asymmetries. A model introducing interference of pion and a1-Reggeon exchanges has been successful in reproducing the experimental data. During the RHIC experiment in year 2015, RHIC delivered polarized proton collisions with Au and Al nuclei for the first time, enabling the exploration of the mechanism of transverse single-spin asymmetries with nuclear collisions. The observed asymmetries showed surprisingly strong A-dependence in the inclusive forward neutron production, while the existing framework which was successfull in p+p only predicts moderate A- dependence. Thus the observed data are absolutely unexpected and unpredicted. In this report, experimental and theoretical efforts are discussed to disentangle the observed A-dependence using somewhat semi-inclusive type measurements and Monte-Carlo study, respectively.

Prospects of developing polarized neutron optics on the basis of new method for improving polarizing coatings

A method for improving polarizing neutron coatings by means of antibarrier interlayers has been suggested in PNPI, and the flip ratio for the reflected beam has been increased by an order as compared to prior coatings. In this paper the tasks to be solved for developing polarized neutron optics are formulated, and the factors that can restrict the efficiency of the polarizing devices on the basis of the improved coatings are discussed. The formulas for the polarizing efficiency which take account of the probabilities of both non-spin-flip (NSF) and spin-flip (SF) reflections are obtained, including the case of multiple reflections; the effect of scattering at magnetic inhomogeneities and that of non-collinearity of the magnetization to the magnetizing field are separated. In the case of multiple reflections the SF probability in the last reflection is shown to play a significant or even a primary role in determining the polarizing efficiency. New neutron instrumentation can be designed with improved polarizing devices, and the possibility to efficiently polarize and analyze very cold or even ultracold neutrons may appear. Thence arises the importance of studying oxidation kinetics and stability of Ti and Co nanolayers.

Chimiothérapie (CT) de maintenance par bevacizumab/pemetrexed chez des patients (pts) avec un cancer bronchique non à petites cellules non épidermoïdes (CBNPCne) avancé : survie globale actualisée de l’essai randomisé de phase III AVAPERL

For a selective excitation of spin-flip strength, (6Li,6He) reactions can be a useful tool, because: a) 6Li and 6He have ground-state spin and parity values Jπ = 1+ and 0+, respectively; b) 6He has no particle-stable excited state, thus effects from ejectile excitation can be avoided. The (6Li,6He) experiments were performed at 100 MeV/nucleon, at which incident energy the reaction is expected to be single-step dominant. Through the analysis of the good-resolution (6Li,6He) data on 13C, 26Mg, 27Al and 58Ni targets, the evidence for a proportional relationship between 0° cross sections and the corresponding Gamow-Teller transition strengths is examined. Suppression of non-spin flip excitations is discussed based on the hindrance of the excitation of the isobaric analog state.

Speromagnetism at the ferrimagnetic compensation point in an Fe64Er19B17 metallic glass — the head of a dandelion, or the spokes of a wheel?

Magnetization and neutron scattering measurements with polarization analysis were made on an Fe64Er19B17 glass in an applied field of 2 Tesla to investigate its ferrimagnetic compensation at Tcomp = 112±2 K. The measurements prove that the glass has a non-collinear magnetic structure. The two non-spin flip cross-sections interchange between 100 K and 125 K, showing the magnetic structure flips at Tcomp. They are identical at 112 K where the forward limit of the spin flip crosssections, |dσ±∓/dΘ|Q=0, was also greatest. We discuss these observations, and compare competing models for the magnetic structure of the glass at Tcomp.

Magnetic correlations in the spin iceHo2−xYxTi2O7as revealed by neutron polarization analysis

We present single-crystal neutron-diffuse-scattering measurements with polarization analysis on the spin ice Ho2-xYxTi2O7 (x = 0, 0.3, and 1). At 2 K, the spin flip scattering patterns are typical of a nearest-neighbor spin ice and the effects of Y doping are seen in the width of the pinch points. At low-T, the spin flip patterns are characteristic of a dipolar spin ice and are unaffected by Y doping. In the non-spin flip channel, we observe the signature of strong antiferromagnetic correlations at the same T as the dipolar spin ice correlations appear in the spin flip channel. These experimental observations have been well reproduced by our Monte Carlo simulations.

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho2−xYxTi2O7 (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1−10) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi2O7 and compare our results with pure spin ice Ho2Ti2O7. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.

Magnetic correlations in pyrochlore spin ice as probed by polarized neutron scattering

Polarized neutron diffuse scattering experiments have been carried out on (1–10) and (111) oriented single crystals of the pyrochlore spin ice [1] compound Ho2Ti2O7 in order to investigate the complex spin correlations in this material at low temperature. The incident neutron beam was polarized perpendicular to the horizontal scattering plane. Thus, the corresponding components of the spin correlations in and out of the scattering plane can be probed with spin-flip (SF) and non-spin-flip (NSF) scattering respectively via neutron polarization analysis. A strong anisotropy of the in-plane and out-of-plane spin scattering has been observed in both samples. The temperature dependence of magnetic diffuse scattering has also been measured. Here we concentrate on reporting the experimental results. In addition, a brief discussion of the possible nature of spin correlations in this pyrochlore spin ice will also be given.

Polarized neutron reflectivity of dilute magnetic semiconductors

We report on a polarized neutron reflectivity investigation of the magnetization in Ge-based dilute magnetic semiconductors. We could observe a net magnetization from the splitting of the non-spin flip reflectivity patterns, which measure the magnetic moment parallel and antiparallel to the applied field. This contrast is visible at 50 K, at remanence and it is pronounced at higher fields even at 250 K for an inhomogeneous specimen. For a homogeneous sample the magnetic variation is visible only at 50 K and above 1.0 kOe. Thus, polarized neutron reflectivity can be a useful tool for investigating the magnetism in homogeneous and inhomogeneous thin film magnetic semiconductors.

Measurement and simulation of polarized neutron reflectivity and off-specular scattering from evolving magnetic domain structure in Co/Cu multilayers

We report on the measurement and simulation of the field-dependent polarized neutron reflectivity (specular) and scattering (off-specular) for an antiferromagnetically coupled Co/Cu multilayer (ML) with number of bilayers N = 40 . The first-order Bragg peak is visible in the non-spin flip (NSF) channels representing the bilayer periodicity. We observe off-specular intensity around the AF Bragg peak at the half-order position in both SF and NSF channels. These Bragg sheets are most intense at lower fields and gradually disappearing at higher fields as the system attains saturation. We attribute the Bragg sheets to vertically correlated AF domains along the ML stack which get bigger in size with field strength. Those interpretations are confirmed by simulation of the data within the distorted wave born approximation (DWBA).

Neutron polarization analysis of nanostructured ZnFe 2O 4

We have investigated nanostructured ZnFe 2O 4 of mean particle size d ∼18(2) nm over the temperature range 24–300 K using neutron polarization analysis. Separation of the spin flip and non-spin flip neutron scattering demonstrates that the diffuse peak which appears at (0.66,0.66,0) at 24 K is of magnetic origin and is elastic. This antiferromagnetic diffuse scattering is likely to depend on the size of the nanoparticles and their fluctuations. Comparison of zero field and field cooled neutron depolarization measurements reveal the formation of oriented magnetic clusters at low temperatures in agreement with magnetization measurements.

Optimising neutron polarisers—measuring a single cross-section

This article is part of a series of works exploring the optimisation of neutron polarisation analysis measurements. It deals with measurements of individual spin flip and non-spin flip neutron scattering cross-sections. An instrumental quality factor is presented. The optimum effective thickness for gaseous spin polarised 3He transmission filters is derived and presented. Cu 2MnAl Heusler alloy polarising monochromators and supermirror devices are considered using the quality factor. Absolute comparisons are made between these different types of polarisers. The effect of instrumental background is calculated for a wide range of experimental situations. Even very small backgrounds can have a very large effect on the quality of measurements achievable indicating that great attention must be paid to background reduction on polarisation analysis instruments.

( 6Li, 6He) reaction at 100 MeV/nucleon as a probe of spin-excitation strengths

For a selective excitation of spin-flip strength, ( 6Li, 6He) reactions can be a useful tool, because: a) 6Li and 6He have ground-state spin and parity values J π = 1 + and 0 +, respectively; b) 6He has no particle-stable excited state, thus effects from ejectile excitation can be avoided. The ( 6Li, 6He) experiments were performed at 100 MeV/nucleon, at which incident energy the reaction is expected to be single-step dominant. Through the analysis of the good-resolution ( 6Li, 6He) data on 13C, 26Mg, 27Al and 58Ni targets, the evidence for a proportional relationship between 0° cross sections and the corresponding Gamow-Teller transition strengths is examined. Suppression of non-spin flip excitations is discussed based on the hindrance of the excitation of the isobaric analog state.

A new technique for polarized neutron diffraction

We have developed a new technique for polarized neutron diffraction utilizing a two-dimensional, area-sensitive detector. An area-sensitive detector, used in conjunction with a transmission polarizer in the scattered beam, allows us to simultaneously measure non-spin-flip (NSF) and spin-flip (SF) scattering processes and therefore increases the data collection rate by a factor of two.